1. Field of the Invention
The present invention relates to cooling devices in large buildings and, in particular, concerns a large diameter low speed fan that can be used to slowly circulate a large volume of air in a uniform manner throughout a building so as to facilitate cooling of individuals or animals located in the building.
2. Description of the Related Art
People who work in large structures such as warehouses and manufacturing plants are routinely exposed to working conditions that range from being uncomfortable to hazardous. On a hot day, the inside air temperature can reach a point where a person is unable to maintain a healthy body temperature. Moreover, many activities that occur in these environments, such as welding or operating internal combustion engines, create airborne contaminants that can be deleterious to those exposed. The effects of airborne contaminants are magnified to an even greater extent if the area is not properly vented.
The problem of cooling large structures cannot always be solved using conventional air-conditioning methods. In particular, the large volume of air that is enclosed within a large structure would require powerful air conditioning devices to be effective. If such devices were used, the operating costs would be substantial. The cost of operating large air conditioning devices would be even greater if large doors where routinely left in an open state or if ventilation of outside air was required.
In general, fans are commonly used to provide some degree of cooling when air conditioning is not feasible. A typical fan consists of a plurality of pitched blades radially positioned on a rotatable hub. The tip-to-tip diameter of such fans typically range from 3 feet up to 5 feet.
When a typical fan rotates under the influence of a motor at higher rotational speeds, a pressure differential is created between the air near the fan blades and the surrounding air, causing a generally conical flow of air that is directed along the fan's axis of rotation. The conical shape combined with drag forces acting at the boundary of the moving mass of air cause the airflow pattern to flare out in a diffusive manner at downstream locations. As a consequence, the ability of these types of fans to provide effective and efficient cooling can be limited for individuals located at a distance from the fan.
In particular, the effectiveness of a fan is based on the principle of evaporation. When the temperature of a human body increases beyond a threshold level, the body responds by perspiring. Through the process of evaporation, the more energetic molecules comprising the perspiration are released into the surrounding air, thus resulting in an overall decrease in the thermal energy of the exterior of the individual's body. The decrease in thermal energy due to evaporation serves to offset positive sources of thermal energy in the individual's body including metabolic activity and heat conduction with surrounding high temperature air.
The rate of evaporative heat loss is highly dependent on the relative humidity of the surrounding air. If the surrounding air is motionless, then a layer of saturated air usually forms near the surface of the individual's skin which dramatically decreases the rate of evaporative heat loss as it prevents the evaporation from the individual's body. At this point, perspiration builds up causing the body to break out into a sweat. The lack of an effective heat loss mechanism results in the body temperature increasing beyond a desired level.
The airflow created by a fan helps to break up the saturated air near the surface of a person's skin and replace it with unsaturated air. This effectively allows the process of evaporation to continue for extended periods of time. The desired result is that the body temperature remains at a comfortable level.
In large buildings, the conventional strategy for cooling individuals has been to employ many commonly available small diameter indoor fans. Small diameter fans have been favored over large diameter fans primarily because of physical constraints. In particular, large diameter fans require specially constructed high-strength light-weight blades that can withstand large stresses caused by significant gravitational moments that increase with an increasing blade length to width aspect ratio. In addition, the fact that the rotational inertia of the fan increases with the square of the diameter requires the use of high torque producing gear reduction mechanisms. Moreover, drive-train components are highly susceptible to mechanical failure due to the very large torques produced by conventional electric motors during their startup phase.
A drawback of using a conventional small diameter fan to create a continuous flow of air is that the resulting airflow dramatically decreases at downstream locations. This is due to the conical nature of the airflow combined with the relatively small mass of air that is contained in the airflow in comparison to resistive drag forces acting at the edge of the cone. To achieve a sufficient airflow in a large non-insulated building, a very large number of small diameter fans would be required. However, the large amount of electrical power required by the simultaneous use of these devices in great numbers negates their advantage as an inexpensive cooling system. Moreover, the use of many fans in an enclosed space can also result in increased air turbulence that can actually decrease the air flow in the building thereby decreasing the cooling effect of the fan.
To achieve a sufficient airflow in large buildings without relying on an impractically large number of small diameter fans, a small number of small diameter fans are typically operated at very high speeds. However, although these types of fans are capable of displacing a large amount of air in a relatively small amount of time, they do so in an undesirable manner. In particular, a small high speed fan operates by moving a relatively small amount of air at a relatively high speed. Consequently, the speed of the airflow adjacent the fan and the level of noise produced are both very high. Furthermore, lighter weight objects, such as papers, may get displaced by the high speed air flow, thus causing a major disruption to the work environment.
Another problem with high speed fans is that they are inefficient at entraining a large enclosed volume of air in a steady continuous airflow pattern. In particular, assuming a best case scenario of laminar airflow, the power consumption of a fan is proportional to the cube of the airspeed produced by the fan. Consequently, an electrically driven high speed fan having a corresponding high speed airflow consumes electrical power at a relatively large rate. Furthermore, the effects of turbulence, which become more pronounced as the speed of the airflow increases, cause the translational kinetic energy associated with the airflow of a high speed fan to be dissipated within a relatively small volume of air. Consequently, even though a relatively large amount of electrical power is consumed by the high speed fan, negligible airflows are produced at locations that are distant from the fan.
To overcome insufficient airflow problems, larger numbers of high speed fans are sometimes used. However, this solution increases the ambient noise and operating costs even further. In addition, regions of fast moving air are expanded, thus increasing the risk of injury to exposed individuals. In particular, if the air is moving fast enough, foreign objects can become airborne, thus causing a hazardous situation. Papers and other light objects can also be greatly effected. Moreover, if the air temperature is above the skin temperature of an individual, then air moving faster than what is needed to break up the boundary layer actually reduces the cooling effect due to the increased rate of heat flow from the higher temperature air to the lower temperature skin of the individual.
In addition to cooling, fans are also relied upon in ventilation systems that serve to remove airborne contaminants such as exhaust or smoke. Typical ventilation systems consist of a set of high speed fans located at the perimeter of the structure. However, the previously mentioned problems of high speed fans apply to high speed ventilation fans. The most serious problem is that some areas inside the structure are not properly ventilated.
To improve ventilation, high speed indoor fans are sometimes used to distribute contaminants throughout the entire volume of a structure. However, the same limitations of high speed indoor fan systems described earlier apply to the problem of ventilation. In particular, high speed indoor fans are loud, inefficient, provide an insufficient airflow to some regions, and provide an undesirably large airflow to others.
From the foregoing, it will be appreciated that there is a need for a cost efficient cooling device that can be effectively operated in large buildings. Furthermore, there is a need for such a device that is very efficient and does not disrupt the work environment with excessive noise or high speed airflows. Furthermore, there is a need for such a device that will dilute concentrated pockets of contaminated air contained within the structure more uniformly, thus providing optimal ventilation to the structure when used in conjunction with a conventional ventilation system.